To meet the growing need for ever increasing digital data bandwidth for new subscriber services (e.g., high-data-rate internet service, video telephony, and high definition TV (HDTV)), telephone companies are looking towards the use of very-high-speed digital subscriber lines (VDSL) as a means to carry such data into the home over the existing copper wires used by plain old telephone service (POTS). VDSL may carry digital data at bit rates up to 52 Mbps using quadrature amplitude modulation (QAM), with modulation constellations of up to 256 symbol points and symbol rates up to 6480 kbaud. The signals are carried within a frequency range reaching 30 MHz, which is above the frequency band used by POTS on the same copper wire pair. Due to the high loss of the copper telephone wires at these frequencies, the VSDL signals are carried on the telephone wires only over the "last mile" (i.e., the last segment of copper wire between a central location and the user's premises). The high-speed digital data is typically carried over a fiber-optic cable network from high-data-rate digital servers to a distribution point where this last mile of copper wire begins. As shown in FIG. 1, the VDSL copper wire is connected to the fiber optic cable 2 by an interface known as a VDSL Transmission Unit, for example, an optical VDSL terminal unit, VTU 4 (VTU-O). At the subscriber's premises, the VDSL line is terminated and interfaced to the subscriber's terminal equipment 22 (e.g., computers, video telephones, HDTV) by an interface known as a VDSL Terminal Unit, for example, a remote VTU 22 (VTU-R).
Within the VDSL frequency range, it has been found that the loop attenuation function for twisted-pair transmission line, in dB/mile, varies as f for frequencies above 300 kHz on a 26-gauge twisted-pair line, and for frequencies above 200 kHz on a 24-gauge line (see Jean-Jacques Werner, "The HDSL Environment", IEEE Journal on Selected Areas in Communications, Vol. 9, No. 6, August 1991, pp. 785-800). This result, as confirmed by simulations of various VDSL test cases, shows that the line attenuation at 30 MHz may be as much as 200 dB more severe than at audio frequencies. In addition, twisted-pair telephone lines may include bridged taps, which are open-circuited twisted pairs that are connected in shunt with working twisted pairs. Bridged taps are sometimes added to telephone lines to provide plant flexibility for future additions and changes in service demands. A bridged tap causes delayed reflections of the transmitted VDSL signal back from the end of the bridged tap and into the main transmission line. These delayed reflections add to the direct signal propagating from transmitter to receiver, resulting in a multipath effect that may cause deep nulls in frequency bands where the reflected signal from the bridged tap has opposite phase sense from the direct path signal, as is illustrated in FIG. 2. A VDSL signal, with typical bandwidth of a few Megahertz, will suffer severe frequency distortion as a result of the great difference in attenuation from the low to high edges of its transmission band due to the f frequency dependence of the line attenuation. This may be further compounded if the signal's transmission band overlaps a bridged tap null. This frequency distortion will cause intersymbol interference which will degrade the probability of bit error and may result in inadequate quality of service.
In principle, an adaptive equalizer at the receiver can correct the frequency distortion caused by the VDSL transmission line while correcting for other distortions that may occur on the received signal. However, in practice, the very large frequency-dependent attenuations in a VDSL transmission line, along with other distortions, often prove to be too much for a reasonably sized equalizer to compensate in a short adaptation time. Therefore, it is advantageous to compensate these frequency distortions by pre-emphasis of the attenuated frequencies prior to transmission, in exact proportion to the amount of attenuation at each frequency, so that the attenuation function of the signal received at the receiver is reasonably flat. The pre-emphasis need not be exact, but to the extent that it can compensate for most of the fixed frequency-dependent attenuation, it places a much smaller processing load on the equalizer, enabling a shorter, simpler and less costly equalizer to be used.
Thus, what is needed is a method and apparatus for automatically compensating the transmission of a wide band data signal over a stable but occasionally varying transmission medium, such as a transmission line, for the distorting effects of non-uniform attenuation over the available bandwidth of the transmission medium when the frequency band of the data signal may from time to time change within that available bandwidth. This can be accomplished by pre-emphasizing the most attenuated parts of the signal spectrum prior to transmission, so that the received signal has a flat spectrum that will not cause frequency distortion.